The effects of alkali metal on structure of manganese oxide supported on SBA-15 for application in the toluene catalytic oxidation

The effects of alkali metal on the toluene catalytic oxidation were studied over the manganese oxide supported on SBA-15. Their properties were characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), diffuse reflectance spectra (DRS) and Fourier transform infrared (FTIR) techniques. It was found that the K/Mn molar ratios over the range of 0.1/1–0.3/1 greatly influenced the structure of the catalyst and the activity for toluene oxidation. When the K/Mn molar ratio was lower than 0.125/1, it led to a decrease in the dispersion of metal oxide particles and the reducibility, resulting in low reactivity for toluene oxidation. Increasing the K/Mn molar ratios to 0.15/1–0.18/1 resulted in the transformation of parts of MnO2 into Mn2O3, and accompanying with the high dispersion of metal oxides and better reducibility. The catalytic activity was obviously increased when the K/Mn molar ratio was 0.15/1–0.18/1. The reactivity of the sample with K/Mn above 0.18/1, however, declined sharply due to the blocking of active sites by K. Two important adsorption sites existed on the KMn/SBA-15 system, in which one was isolated OH groups on the SBA-15 and the other was the activated Si–O–Mn bond under the effect of K.

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► K plays an important role in the structure and reactivity of Mn/SBA-15.
► A proper K/Mn molar ratio leads parts of MnO2 to being transformed into Mn2O3.
► A proper content of K enhances the reducibility of Mn/SBA-15 and its reactivity.
► Si–O–Mn bond of catalysts becomes to be available for toluene adsorption on the KMn/SBA-15.